S-Adenosylmethionine decarboxylase is a critical enzyme in the polyamine biosynthetic pathway and depends on a pyruvoyl group for the decarboxylation process. Inhibitors of this enzyme have potential as cancer chemotherapeutic drugs and for treating various parasitic infections. The crystal structures of the enzyme with various inhibitors at the active site have been determined previously and have shown that the adenine base of the ligands adopts an unusual syn conformation during interaction with the enzyme. For example, it is known that 8-substitution on adenine rings causes the nucleotide to adopt a syn conformation in solution (37-40). In the syn conformation, the adenine base stacks between the F223 and F7 residues of AdoMetDC. S-Adenosylmethionine decarboxylase (AdoMetDC) is a pyruvoyl dependent decarboxylase and a critical enzyme in the polyamine biosynthetic pathway which is found in all species (1-4). The polyamines putrescine, spermidine and spermine are essential for cell growth and play an important role in cell proliferation and differentiation (5-7). Polyamines have been found to be elevated in various types of cancer including non small cell lung cancer, prostate cancer, melanoma, and pancreatic cancer (8, 9). Polyamine levels in cells depend on the polyamine synthetic and catabolic pathways as well as on import and export of polyamines across the cellular membrane. Altering regulation of the key enzymes in the polyamine pathway is a therapeutic strategy for treatment of various types of cancers. AdoMetDC catalyzes the conversion of S-adenosylmethionine (AdoMet) to decarboxylated-S-adenosylmethionine (dcAdoMet), which then donates the aminopropyl group to putrescine or spermidine to form spermidine and spermine, respectively. AdoMetDC is at a key branch point in the pathway and its action commits AdoMet to polyamine biosynthesis and removes it from the pool available for methyl donation.
Attempts to regulate polyamine levels, have resulted in the development of inhibitors that target the biosynthetic enzymes ornithine decarboxylase (ODC) (10), AdoMetDC and the catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) (11). The best-known inhibitor of ODC is α-difluoromethylornithine (DFMO) which irreversibly inactivates the enzyme. The success of DFMO in cancer therapy has been limited as the cells compensate for the decreased synthesis of polyamines through increased cellular uptake of polyamines (12). DFMO is currently being investigated as a chemopreventive agent against carcinogenesis (13-15). The development of drugs to inhibit AdoMetDC started with the competitive inhibitor methylglyoxal bis(guanylhydrazone) (MGBG) which is similar to spermidine in structure (16). Use of MGBG caused extreme toxicity in humans and many analogues of MGBG were developed in attempts to decrease the toxicity. One such AdoMet inhibitor that resulted was 4-amidinoindan-1-one-2′-amidinohydrazone (CGP48664A) which has gone on to clinical trials as a cancer chemotherapeutic agent (17). Alternately, inhibitors like MHZPA, MAOEA and MHZEA that are structural analogues of the natural substrate were developed. These compounds inactivate AdoMetDC by forming a Schiff base to the active site pyruvoyl group (18). Another known nucleoside inhibitor of AdoMetDC is 5′-(18)-5′-deoxyadenosine. This was designed as an enzyme-activated irreversible inhibitor (19) but subsequent experiments showed that it acted via by a transamination of the pyruvate prosthetic group (18).
The crystal structure of AdoMetDC and its S68A and H243A mutants were solved to understand the mechanisms of decarboxylation and autoprocessing (20-22). The crystal structures of AdoMetDC with inhibitors like MAOEA, MHZPA and MeAdoMet have been solved previously (23). These structures show that the adenine base of the inhibitors assumes an unusual syn conformation in the active site.